How autowiring works in Spring?

0 votes

Say I have a service class called UserServiceImpl that implements UserService interface.

How can this be @Autowired?

And in my Controllers, how would I instantiate an instance of this service?

Would I just do the following?

UserService userService = new UserServiceImpl();
Jan 11 in Java by Sushmita
• 6,840 points
42 views

3 answers to this question.

Your answer

Your name to display (optional):
Privacy: Your email address will only be used for sending these notifications.
0 votes

Depends on whether you went the annotations route or the bean XML definition route.

Say you had the beans defined in your applicationContext.xml:

<beans ...>

    <bean id="userService" class="com.foo.UserServiceImpl"/>

    <bean id="fooController" class="com.foo.FooController"/>

</beans>

The autowiring happens when the application starts up. So, in fooController, which for arguments sake wants to use the UserServiceImpl class, you'd annotate it as follows:

public class FooController {

    // You could also annotate the setUserService method instead of this
    @Autowired
    private UserService userService;

    // rest of class goes here
}

When it sees @Autowired, Spring will look for a class that matches the property in the applicationContext, and inject it automatically. If you have more than 1 UserService bean, then you'll have to qualify which one it should use.

answered Jan 11 by Daisy
• 8,040 points
0 votes

Depends on whether you went the annotations route or the bean XML definition route.

Say you had the beans defined in your applicationContext.xml:

<beans ...>

    <bean id="userService" class="com.foo.UserServiceImpl"/>

    <bean id="fooController" class="com.foo.FooController"/>

</beans>

The autowiring happens when the application starts up. So, in fooController, which for arguments sake wants to use the UserServiceImpl class, you'd annotate it as follows:

public class FooController {

    // You could also annotate the setUserService method instead of this
    @Autowired
    private UserService userService;

    // rest of class goes here
}

When it sees @Autowired, Spring will look for a class that matches the property in the applicationContext, and inject it automatically. If you have more than 1 UserService bean, then you'll have to qualify which one it should use.

answered Jan 11 by Daisy
• 8,040 points
0 votes

First, and most important - all Spring beans are managed - they "live" inside a container, called "application context".

Second, each application has an entry point to that context. Web applications have a Servlet, JSFuses a el-resolver, etc. Also, there is a place where the application context is bootstrapped and all beans - autowired. In web applications this can be a startup listener.

Autowiring happens by placing an instance of one bean into the desired field in an instance of another bean. Both classes should be beans, i.e. they should be defined to live in the application context.

What is "living" in the application context? This means that the context instantiates the objects, not you. I.e. - you never make new UserServiceImpl() - the container finds each injection point and sets an instance there.

In your controllers, you just have the following:

@Controller // Defines that this class is a spring bean
@RequestMapping("/users")
public class SomeController {

    // Tells the application context to inject an instance of UserService here
    @Autowired
    private UserService userService;

    @RequestMapping("/login")
    public void login(@RequestParam("username") String username,
           @RequestParam("password") String password) {

        // The UserServiceImpl is already injected and you can use it
        userService.login(username, password);

    }
}

A few notes:

  • In your applicationContext.xml you should enable the <context:component-scan> so that classes are scanned for the @Controller, @Service, etc. annotations.
  • The entry point for a Spring-MVC application is the DispatcherServlet, but it is hidden from you, and hence the direct interaction and bootstrapping of the application context happens behind the scene.
  • UserServiceImpl should also be defined as bean - either using <bean id=".." class=".."> or using the @Service annotation. Since it will be the only implementor of UserService, it will be injected.
  • Apart from the @Autowired annotation, Spring can use XML-configurable autowiring. In that case all fields that have a name or type that matches with an existing bean automatically get a bean injected. In fact, that was the initial idea of autowiring - to have fields injected with dependencies without any configuration. Other annotations like @Inject, @Resource can also be used.
answered Jan 11 by developer_1
• 3,280 points

Related Questions In Java

0 votes
1 answer

I am learning looping statements. Can you tell me how 'for-each' works in Java?

While programming we often write code that ...READ MORE

answered Apr 17, 2018 in Java by Rishabh
• 3,520 points
32 views
0 votes
0 answers

Can anyone explain Autowiring in Spring ?

Say I have a service class called UserServiceImpl that ...READ MORE

Jan 11 in Java by Sushmita
• 6,840 points
26 views
+5 votes
3 answers

How to execute a python file with few arguments in java?

You can use Java Runtime.exec() to run python script, ...READ MORE

answered Mar 27, 2018 in Java by DragonLord999
• 8,360 points

edited Nov 6, 2018 by Omkar 5,209 views
0 votes
1 answer

How can I convert a String variable to a primitive int in Java

You can convert a String to int by using a function: int ...READ MORE

answered Apr 12, 2018 in Java by sharth
• 3,310 points
55 views
0 votes
1 answer

Bean life cycle in Spring Bean Factory Container

Bean life cycle in Spring Bean Factory ...READ MORE

answered Aug 29, 2018 in Java by code.reaper12
• 3,450 points
233 views
0 votes
1 answer

How to download a file from spring controllers?

@RequestMapping(value = "/files/{file_name}", method = RequestMethod.GET) public void ...READ MORE

answered Sep 4, 2018 in Java by code.reaper12
• 3,450 points
73 views
0 votes
1 answer

Understanding Spring @Autowired usage

TL;DR The @Autowired annotation spares you the need ...READ MORE

answered Nov 30, 2018 in Java by Sushmita
• 6,840 points
192 views
0 votes
1 answer

Why is my Spring @Autowired field null?

If you are not coding a web ...READ MORE

answered Dec 27, 2018 in Java by Daisy
• 8,040 points
1,164 views
0 votes
5 answers

How to compare Strings in Java?

String fooString1 = new String("foo"); String fooString2 = ...READ MORE

answered Jul 12, 2018 in Java by Daisy
• 8,040 points
87 views
0 votes
2 answers

How to create a 2-D array in java?

int[][] multi = new int[5][]; multi[0] = new ...READ MORE

answered Jul 16, 2018 in Java by Daisy
• 8,040 points
25 views

© 2018 Brain4ce Education Solutions Pvt. Ltd. All rights Reserved.
"PMP®","PMI®", "PMI-ACP®" and "PMBOK®" are registered marks of the Project Management Institute, Inc. MongoDB®, Mongo and the leaf logo are the registered trademarks of MongoDB, Inc.